Do not run unneeded subsystems on collator and its alongside node (#3061)

Currently, collators and their alongside nodes spin up a full-scale
overseer running a bunch of subsystems that are not needed if the node
is not a validator. That was considered to be harmless; however, we've
got problems with unused subsystems getting stalled for a reason not
currently known, resulting in the overseer exiting and bringing down the
whole node.

This PR aims to only run needed subsystems on such nodes, replacing the
rest with `DummySubsystem`.

It also enables collator-optimized availability recovery subsystem
implementation.

Partially solves #1730.

polkadot/cli/src/command.rs

@@ -299,7 +299,7 @@ pub fn run() -> Result<()> {
 	match &cli.subcommand {
 		None => run_node_inner(
 			cli,
-			service::RealOverseerGen,
+			service::ValidatorOverseerGen,
 			None,
 			polkadot_node_metrics::logger_hook(),
 		),